1. Field of the Invention
The present invention relates to an anti-skid apparatus for use in an automotive vehicle, and more particularly, to an anti-skid apparatus for installation in a vehicle braking system between a master cylinder and wheel brake cylinders to prevent the vehicle wheels from locking in braking operation.
2. Description of the Prior Art
As one of such anti-skid apparatus as described above, Japanese Patent Publication for Opposition in 1984 under Publication No. 49-28307 discloses an anti-skid apparatus wherein an input control valve and an output control valve are disposed in parallel with each other in a primary fluid passage connecting a master cylinder and a wheel brake cylinder, and is disposed a fluid pump for pumping a brake fluid from a reservoir connected to the output control valve and returning the pumped brake fluid via a pump passage to the primary fluid passage at a portion thereof between the the master cylinder and the input control valve.
In the above-described prior apparatus, a hydraulic pressure in the wheel brake cylinder is alternately increased and decreased in response to operation of the changeover valve during its anti-skid operation. When the hydraulic pressure in the wheel brake cylinder is to be decreased with the input control valve in its closed position, the hydraulic pressure in the above-described portion of the primary fluid passage is increased, since the pumped brake fluid is supplied to that portion where the hydraulic pressure is increased in response to depression of the brake pedal. Accordingly, when the hydraulic pressure in the wheel brake cylinder is to be increased with the input valve in its open position, the increasing speed of the hydraulic pressure in the wheel brake cylinder becomes high, so that the hydraulic pressure therein is increased exceedingly. To prevent this extreme rise of the hydraulic pressure in the wheel brake cylinder, it may be useful to provide the primary fluid passage with a throttle or a restrictor for reducing the hydraulic pressure, as in Japanese Patent Application laid open in 1977 under Publication No. 52-12869, which discloses an anti-skid apparatus wherein a solenoid operated changeover valve selectively placed in one of a first operating position for communication of the wheel brake cylinder with the master cylinder, and a second operating position for communication of the wheel brake cylinder with the reservoir, is disposed in the primary fluid passage, in lieu of the input and output control valves, and wherein a plurality of restrictors are disposed in the primary fluid passage. However, in the normal braking operation, the flow of the brake fluid from the master cylinder to the wheel brake cylinder will also be restricted by the restrictors, so that delay in braking operation will be caused.
In the anti-skid apparatus disclosed in Japanese Patent Application laid open in 1986 under Publication No. 61-282158, a cut-off valve is disposed in the primary fluid passage between the master cylinder and the changeover valve. The cut-off valve is provided with a restrictor for restricting the flow of the brake fluid when the cut-off valve is in its closed position, and associated with a piston one side of which is applied with the hydraulic pressure in the primary fluid passage between the master cylinder and the changeover valve, and the other side of which is applied with the hydraulic pressure between the changeover valve and the wheel brake cylinder. This piston is arranged to normally hold the cut-off valve in its open position and activate the cut-off valve in its closed position when the hydraulic pressure on the side of the wheel brake cylinder becomes lower than the hydraulic pressure on the side of the master cylinder by more than a predetermined value. Accordingly, in the anti-skid operation, the increasing speed of the hydraulic pressure in the wheel brake cylinder may be reduced. In this anti-skid apparatus, however, the restrictor might not be able to restrict the flow of the brake fluid effectively in the anti-skid operation, when the difference between the hydraulic pressure on the side of the wheel brake cylinder and that on the side of the master cylinder is small, since the cut-off valve is designed to keep a sufficient passage for the brake fluid in normal braking operation.